System
class System
kotlin.Any | |
↳ | java.lang.System |
The System
class contains several useful class fields and methods. It cannot be instantiated.
Among the facilities provided by the System
class are standard input, standard output, and error output streams; access to externally defined properties and environment variables; a means of loading files and libraries; and a utility method for quickly copying a portion of an array.
Summary
Public methods | |
---|---|
static Unit |
Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array. |
static String? |
clearProperty(key: String) Removes the system property indicated by the specified key. |
static Console? |
console() Returns the unique |
static Long |
Returns the current time in milliseconds. |
static Unit |
Terminates the currently running Java Virtual Machine. |
static Unit |
gc() Runs the garbage collector. |
static Properties |
Determines the current system properties. |
static String? |
getProperty(key: String) Gets the system property indicated by the specified key. |
static String? |
getProperty(key: String, def: String?) Gets the system property indicated by the specified key. |
static SecurityManager? |
Always returns |
static String? |
Gets the value of the specified environment variable. |
static MutableMap<String!, String!> |
getenv() Returns an unmodifiable string map view of the current system environment. |
static Int |
identityHashCode(x: Any?) Returns the same hash code for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode(). |
static Channel? |
Returns the channel inherited from the entity that created this Java virtual machine. |
static String |
Returns the system-dependent line separator string. |
static Unit |
Loads the native library specified by the filename argument. |
static Unit |
loadLibrary(libname: String) Loads the native library specified by the |
static String |
mapLibraryName(libname: String) Maps a library name into a platform-specific string representing a native library. |
static Long |
nanoTime() Returns the current value of the running Java Virtual Machine's high-resolution time source, in nanoseconds. |
static Unit |
Runs the finalization methods of any objects pending finalization. |
static Unit |
runFinalizersOnExit(value: Boolean) Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. |
static Unit |
setErr(err: PrintStream?) Reassigns the "standard" error output stream. |
static Unit |
setIn(in: InputStream?) Reassigns the "standard" input stream. |
static Unit |
setOut(out: PrintStream?) Reassigns the "standard" output stream. |
static Unit |
setProperties(props: Properties?) Attempts to set all system properties. |
static String? |
setProperty(key: String, value: String?) Sets the system property indicated by the specified key. |
static Unit |
Throws |
Properties | |
---|---|
static PrintStream! |
The "standard" error output stream. |
static InputStream! |
The "standard" input stream. |
static PrintStream! |
The "standard" output stream. |
Public methods
arraycopy
static fun arraycopy(
src: Any,
srcPos: Int,
dest: Any,
destPos: Int,
length: Int
): Unit
Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array. A subsequence of array components are copied from the source array referenced by src
to the destination array referenced by dest
. The number of components copied is equal to the length
argument. The components at positions srcPos
through srcPos+length-1
in the source array are copied into positions destPos
through destPos+length-1
, respectively, of the destination array.
If the src
and dest
arguments refer to the same array object, then the copying is performed as if the components at positions srcPos
through srcPos+length-1
were first copied to a temporary array with length
components and then the contents of the temporary array were copied into positions destPos
through destPos+length-1
of the destination array.
If dest
is null
, then a NullPointerException
is thrown.
If src
is null
, then a NullPointerException
is thrown and the destination array is not modified.
Otherwise, if any of the following is true, an ArrayStoreException
is thrown and the destination is not modified:
- The
src
argument refers to an object that is not an array. - The
dest
argument refers to an object that is not an array. - The
src
argument anddest
argument refer to arrays whose component types are different primitive types. - The
src
argument refers to an array with a primitive component type and thedest
argument refers to an array with a reference component type. - The
src
argument refers to an array with a reference component type and thedest
argument refers to an array with a primitive component type.
Otherwise, if any of the following is true, an IndexOutOfBoundsException
is thrown and the destination is not modified:
- The
srcPos
argument is negative. - The
destPos
argument is negative. - The
length
argument is negative. srcPos+length
is greater thansrc.length
, the length of the source array.destPos+length
is greater thandest.length
, the length of the destination array.
Otherwise, if any actual component of the source array from position srcPos
through srcPos+length-1
cannot be converted to the component type of the destination array by assignment conversion, an ArrayStoreException
is thrown. In this case, let k be the smallest nonnegative integer less than length such that src[srcPos+
k]
cannot be converted to the component type of the destination array; when the exception is thrown, source array components from positions srcPos
through srcPos+
k-1
will already have been copied to destination array positions destPos
through destPos+
k-1
and no other positions of the destination array will have been modified. (Because of the restrictions already itemized, this paragraph effectively applies only to the situation where both arrays have component types that are reference types.)
Parameters | |
---|---|
src |
Any: the source array. |
srcPos |
Int: starting position in the source array. |
dest |
Any: the destination array. |
destPos |
Int: starting position in the destination data. |
length |
Int: the number of array elements to be copied. |
Exceptions | |
---|---|
java.lang.IndexOutOfBoundsException |
if copying would cause access of data outside array bounds. |
java.lang.ArrayStoreException |
if an element in the src array could not be stored into the dest array because of a type mismatch. |
java.lang.NullPointerException |
if either src or dest is null . |
clearProperty
static fun clearProperty(key: String): String?
Removes the system property indicated by the specified key.
First, if a security manager exists, its SecurityManager.checkPermission
method is called with a PropertyPermission(key, "write")
permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is removed.
Parameters | |
---|---|
key |
String: the name of the system property to be removed. |
Return | |
---|---|
String? |
the previous string value of the system property, or null if there was no property with that key. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property. |
java.lang.NullPointerException |
if key is null . |
java.lang.IllegalArgumentException |
if key is empty. |
console
static fun console(): Console?
Returns the unique Console
object associated with the current Java virtual machine, if any.
Return | |
---|---|
Console? |
The system console, if any, otherwise null. |
currentTimeMillis
static fun currentTimeMillis(): Long
Returns the current time in milliseconds. Note that while the unit of time of the return value is a millisecond, the granularity of the value depends on the underlying operating system and may be larger. For example, many operating systems measure time in units of tens of milliseconds.
See the description of the class Date
for a discussion of slight discrepancies that may arise between "computer time" and coordinated universal time (UTC).
Return | |
---|---|
Long |
the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC. |
See Also
exit
static fun exit(status: Int): Unit
Terminates the currently running Java Virtual Machine. The argument serves as a status code; by convention, a nonzero status code indicates abnormal termination.
This method calls the exit
method in class Runtime
. This method never returns normally.
The call System.exit(n)
is effectively equivalent to the call:
Runtime.getRuntime().exit(n)
Parameters | |
---|---|
status |
Int: exit status. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkExit method doesn't allow exit with the specified status. |
See Also
gc
static fun gc(): Unit
Runs the garbage collector.
Calling the gc
method suggests that the Java Virtual Machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse. When control returns from the method call, the Java Virtual Machine has made a best effort to reclaim space from all discarded objects.
The call System.gc()
is effectively equivalent to the call:
Runtime.getRuntime().gc()
Calling this indiscriminately is likely to severely degrade performance. Intended primarily for testing.
See Also
getProperties
static fun getProperties(): Properties
Determines the current system properties.
The following properties are always provided by the Dalvik VM:
Name | Meaning | Example |
file.separator | java.io.File#separator |
/ |
java.class.path | System class path | . |
java.class.version | (Not useful on Android) | 50.0 |
java.compiler | (Not useful on Android) | Empty |
java.ext.dirs | (Not useful on Android) | Empty |
java.home | Location of the VM on the file system | /apex/com.android.art/ |
java.io.tmpdir | Location of a temporary directory. The location varies by application. See java.io.File#createTempFile |
/data/user/0/com.android.app/cache |
java.library.path | Search path for JNI libraries | /vendor/lib:/system/lib |
java.vendor | Human-readable VM vendor | The Android Project |
java.vendor.url | URL for VM vendor's web site | http://www.android.com/ |
java.version | (Not useful on Android) | 0 |
java.specification.version | VM libraries version | 0.9 |
java.specification.vendor | VM libraries vendor | The Android Project |
java.specification.name | VM libraries name | Dalvik Core Library |
java.vm.version | VM implementation version | 1.2.0 |
java.vm.vendor | VM implementation vendor | The Android Project |
java.vm.name | VM implementation name | Dalvik |
java.vm.specification.version | VM specification version | 0.9 |
java.vm.specification.vendor | VM specification vendor | The Android Project |
java.vm.specification.name | VM specification name | Dalvik Virtual Machine Specification |
line.separator | The system line separator | \n |
os.arch | OS architecture | aarch64 |
os.name | OS (kernel) name | Linux |
os.version | OS (kernel) version | 5.10.98-g6ea688a79989 |
path.separator | See java.io.File#pathSeparator |
: |
user.dir | Base of non-absolute paths | / |
user.home | (Not useful on Android) | Empty |
user.name | (Not useful on Android) | Empty |
Multiple paths in a system property value are separated by the path separator character of the platform.
Note that even if the security manager does not permit the getProperties
operation, it may choose to permit the getProperty(java.lang.String)
operation.
Return | |
---|---|
Properties |
the system properties |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties. |
getProperty
static fun getProperty(key: String): String?
Gets the system property indicated by the specified key.
First, if there is a security manager, its checkPropertyAccess
method is called with the key as its argument. This may result in a SecurityException.
If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties
method.
Parameters | |
---|---|
key |
String: the name of the system property. |
Return | |
---|---|
String? |
the string value of the system property, or null if there is no property with that key. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property. |
java.lang.NullPointerException |
if key is null . |
java.lang.IllegalArgumentException |
if key is empty. |
getProperty
static fun getProperty(
key: String,
def: String?
): String?
Gets the system property indicated by the specified key.
First, if there is a security manager, its checkPropertyAccess
method is called with the key
as its argument.
If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties
method.
Parameters | |
---|---|
key |
String: the name of the system property. |
def |
String?: a default value. |
Return | |
---|---|
String? |
the string value of the system property, or the default value if there is no property with that key. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property. |
java.lang.NullPointerException |
if key is null . |
java.lang.IllegalArgumentException |
if key is empty. |
getSecurityManager
static fun getSecurityManager(): SecurityManager?
Always returns null
in Android
Return | |
---|---|
SecurityManager? |
null in Android |
getenv
static fun getenv(name: String): String?
Gets the value of the specified environment variable. An environment variable is a system-dependent external named value.
If a security manager exists, its java.lang.SecurityManager#checkPermission method is called with a
permission. This may result in a RuntimePermission
("getenv."+name)SecurityException
being thrown. If no exception is thrown the value of the variable name
is returned.
System properties and environment variables are both conceptually mappings between names and values. Both mechanisms can be used to pass user-defined information to a Java process. Environment variables have a more global effect, because they are visible to all descendants of the process which defines them, not just the immediate Java subprocess. They can have subtly different semantics, such as case insensitivity, on different operating systems. For these reasons, environment variables are more likely to have unintended side effects. It is best to use system properties where possible. Environment variables should be used when a global effect is desired, or when an external system interface requires an environment variable (such as PATH
).
On UNIX systems the alphabetic case of name
is typically significant, while on Microsoft Windows systems it is typically not. For example, the expression System.getenv("FOO").equals(System.getenv("foo"))
is likely to be true on Microsoft Windows.
Parameters | |
---|---|
name |
String: the name of the environment variable |
Return | |
---|---|
String? |
the string value of the variable, or null if the variable is not defined in the system environment |
Exceptions | |
---|---|
java.lang.NullPointerException |
if name is null |
java.lang.SecurityException |
if a security manager exists and its java.lang.SecurityManager#checkPermission method doesn't allow access to the environment variable name |
getenv
static fun getenv(): MutableMap<String!, String!>
Returns an unmodifiable string map view of the current system environment. The environment is a system-dependent mapping from names to values which is passed from parent to child processes.
If the system does not support environment variables, an empty map is returned.
The returned map will never contain null keys or values. Attempting to query the presence of a null key or value will throw a NullPointerException
. Attempting to query the presence of a key or value which is not of type String
will throw a ClassCastException
.
The returned map and its collection views may not obey the general contract of the Object#equals
and Object#hashCode
methods.
The returned map is typically case-sensitive on all platforms.
If a security manager exists, its java.lang.SecurityManager#checkPermission method is called with a
permission. This may result in a RuntimePermission
("getenv.*")SecurityException
being thrown.
When passing information to a Java subprocess, system properties are generally preferred over environment variables.
Return | |
---|---|
MutableMap<String!, String!> |
the environment as a map of variable names to values |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its java.lang.SecurityManager#checkPermission method doesn't allow access to the process environment |
identityHashCode
static fun identityHashCode(x: Any?): Int
Returns the same hash code for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode(). The hash code for the null reference is zero.
Parameters | |
---|---|
x |
Any?: object for which the hashCode is to be calculated |
Return | |
---|---|
Int |
the hashCode |
inheritedChannel
static fun inheritedChannel(): Channel?
Returns the channel inherited from the entity that created this Java virtual machine.
This method returns the channel obtained by invoking the inheritedChannel
method of the system-wide default java.nio.channels.spi.SelectorProvider
object.
In addition to the network-oriented channels described in inheritedChannel
, this method may return other kinds of channels in the future.
Return | |
---|---|
Channel? |
The inherited channel, if any, otherwise null. |
Exceptions | |
---|---|
java.io.IOException |
If an I/O error occurs |
java.lang.SecurityException |
If a security manager is present and it does not permit access to the channel. |
lineSeparator
static fun lineSeparator(): String
Returns the system-dependent line separator string. It always returns the same value - the initial value of the system property line.separator
.
On UNIX systems, it returns "\n"
; on Microsoft Windows systems it returns "\r\n"
.
Return | |
---|---|
String |
the system-dependent line separator string |
load
static fun load(filename: String): Unit
Loads the native library specified by the filename argument. The filename argument must be an absolute path name. If the filename argument, when stripped of any platform-specific library prefix, path, and file extension, indicates a library whose name is, for example, L, and a native library called L is statically linked with the VM, then the JNI_OnLoad_L function exported by the library is invoked rather than attempting to load a dynamic library. A filename matching the argument does not have to exist in the file system. See the JNI Specification for more details. Otherwise, the filename argument is mapped to a native library image in an implementation-dependent manner.
The call System.load(name)
is effectively equivalent to the call:
Runtime.getRuntime().load(name)
Parameters | |
---|---|
filename |
String: the file to load. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkLink method doesn't allow loading of the specified dynamic library |
java.lang.UnsatisfiedLinkError |
if either the filename is not an absolute path name, the native library is not statically linked with the VM, or the library cannot be mapped to a native library image by the host system. |
java.lang.NullPointerException |
if filename is null |
loadLibrary
static fun loadLibrary(libname: String): Unit
Loads the native library specified by the libname
argument. The libname
argument must not contain any platform specific prefix, file extension or path. If a native library called libname
is statically linked with the VM, then the JNI_OnLoad_libname
function exported by the library is invoked. See the JNI Specification for more details. Otherwise, the libname argument is loaded from a system library location and mapped to a native library image in an implementation- dependent manner.
The call System.loadLibrary(name)
is effectively equivalent to the call
Runtime.getRuntime().loadLibrary(name)
Parameters | |
---|---|
libname |
String: the name of the library. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkLink method doesn't allow loading of the specified dynamic library |
java.lang.UnsatisfiedLinkError |
if either the libname argument contains a file path, the native library is not statically linked with the VM, or the library cannot be mapped to a native library image by the host system. |
java.lang.NullPointerException |
if libname is null |
mapLibraryName
static fun mapLibraryName(libname: String): String
Maps a library name into a platform-specific string representing a native library.
Parameters | |
---|---|
libname |
String: the name of the library. |
Return | |
---|---|
String |
a platform-dependent native library name. |
Exceptions | |
---|---|
java.lang.NullPointerException |
if libname is null |
nanoTime
static fun nanoTime(): Long
Returns the current value of the running Java Virtual Machine's high-resolution time source, in nanoseconds.
This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary origin time (perhaps in the future, so values may be negative). The same origin is used by all invocations of this method in an instance of a Java virtual machine; other virtual machine instances are likely to use a different origin.
This method provides nanosecond precision, but not necessarily nanosecond resolution (that is, how frequently the value changes) - no guarantees are made except that the resolution is at least as good as that of currentTimeMillis()
.
Differences in successive calls that span greater than approximately 292 years (263 nanoseconds) will not correctly compute elapsed time due to numerical overflow.
The values returned by this method become meaningful only when the difference between two such values, obtained within the same instance of a Java virtual machine, is computed.
For example, to measure how long some code takes to execute:
<code>long startTime = System.nanoTime(); // ... the code being measured ... long estimatedTime = System.nanoTime() - startTime;</code>
To compare two nanoTime values
<code>long t0 = System.nanoTime(); ... long t1 = System.nanoTime();</code>
t1 - t0 < 0
, not t1 < t0
, because of the possibility of numerical overflow.
The value returned by this method does not account for elapsed time during deep sleep. For timekeeping facilities available on Android see android.os.SystemClock
.
Return | |
---|---|
Long |
the current value of the running Java Virtual Machine's high-resolution time source, in nanoseconds |
runFinalization
static fun runFinalization(): Unit
Runs the finalization methods of any objects pending finalization.
Calling this method suggests that the Java Virtual Machine expend effort toward running the finalize
methods of objects that have been found to be discarded but whose finalize
methods have not yet been run. When control returns from the method call, the Java Virtual Machine has made a best effort to complete all outstanding finalizations.
The call System.runFinalization()
is effectively equivalent to the call:
Runtime.getRuntime().runFinalization()
See Also
runFinalizersOnExit
static funrunFinalizersOnExit(value: Boolean): Unit
Deprecated: This method is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock.
Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. By default, finalization on exit is disabled.
If there is a security manager, its checkExit
method is first called with 0 as its argument to ensure the exit is allowed. This could result in a SecurityException.
Parameters | |
---|---|
value |
Boolean: indicating enabling or disabling of finalization |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkExit method doesn't allow the exit. |
setErr
static fun setErr(err: PrintStream?): Unit
Reassigns the "standard" error output stream.
First, if there is a security manager, its checkPermission
method is called with a RuntimePermission("setIO")
permission to see if it's ok to reassign the "standard" error output stream.
Parameters | |
---|---|
err |
PrintStream?: the new standard error output stream. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkPermission method doesn't allow reassigning of the standard error output stream. |
See Also
setIn
static fun setIn(in: InputStream?): Unit
Reassigns the "standard" input stream.
First, if there is a security manager, its checkPermission
method is called with a RuntimePermission("setIO")
permission to see if it's ok to reassign the "standard" input stream.
Parameters | |
---|---|
in |
InputStream?: the new standard input stream. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkPermission method doesn't allow reassigning of the standard input stream. |
See Also
setOut
static fun setOut(out: PrintStream?): Unit
Reassigns the "standard" output stream.
First, if there is a security manager, its checkPermission
method is called with a RuntimePermission("setIO")
permission to see if it's ok to reassign the "standard" output stream.
Parameters | |
---|---|
out |
PrintStream?: the new standard output stream |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkPermission method doesn't allow reassigning of the standard output stream. |
See Also
setProperties
static fun setProperties(props: Properties?): Unit
Attempts to set all system properties. Copies all properties from p
and discards system properties that are read only and cannot be modified. See #getProperty for a list of such properties.
setProperty
static fun setProperty(
key: String,
value: String?
): String?
Sets the system property indicated by the specified key.
First, if a security manager exists, its SecurityManager.checkPermission
method is called with a PropertyPermission(key, "write")
permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is set to the given value.
Parameters | |
---|---|
key |
String: the name of the system property. |
value |
String?: the value of the system property. |
Return | |
---|---|
String? |
the previous value of the system property, or null if it did not have one. |
Exceptions | |
---|---|
java.lang.SecurityException |
if a security manager exists and its checkPermission method doesn't allow setting of the specified property. |
java.lang.NullPointerException |
if key or value is null . |
java.lang.IllegalArgumentException |
if key is empty. |
setSecurityManager
static fun setSecurityManager(s: SecurityManager?): Unit
Throws SecurityException
(except in case sm == null
).
Security managers do not provide a secure environment for executing untrusted code and are unsupported on Android. Untrusted code cannot be safely isolated within a single VM on Android, so this method always throws a SecurityException
when passed a non-null SecurityManager
Parameters | |
---|---|
s |
SecurityManager?: a security manager |
Exceptions | |
---|---|
java.lang.SecurityException |
always, unless sm == null |
Properties
err
static val err: PrintStream!
The "standard" error output stream. This stream is already open and ready to accept output data.
Typically this stream corresponds to display output or another output destination specified by the host environment or user. By convention, this output stream is used to display error messages or other information that should come to the immediate attention of a user even if the principal output stream, the value of the variable out
, has been redirected to a file or other destination that is typically not continuously monitored.
in
static val in: InputStream!
The "standard" input stream. This stream is already open and ready to supply input data. Typically this stream corresponds to keyboard input or another input source specified by the host environment or user.
out
static val out: PrintStream!
The "standard" output stream. This stream is already open and ready to accept output data. Typically this stream corresponds to display output or another output destination specified by the host environment or user.
For simple stand-alone Java applications, a typical way to write a line of output data is:
System.out.println(data)
See the println
methods in class PrintStream
.
See Also
java.io.PrintStream#println()
java.io.PrintStream#println(boolean)
java.io.PrintStream#println(char)
java.io.PrintStream#println(char[])
java.io.PrintStream#println(double)
java.io.PrintStream#println(float)
java.io.PrintStream#println(int)
java.io.PrintStream#println(long)
java.io.PrintStream#println(java.lang.Object)
java.io.PrintStream#println(java.lang.String)